● Medium · CVSS 6.2

How to Fix CVE-2026-3904: CWE-366 Race condition within a thread in glibc

Q: Is CVE-2026-3904 being actively exploited?

Not at the time of writing. It is not listed in CISA's Known Exploited Vulnerabilities catalog. That status can change, so monitor the vendor advisory and the KEV catalog if the system is exposed.

Q: How severe is CVE-2026-3904?

CVSS rates it 6.2 (Medium). Use that score to set your patch priority next to the other items in your queue.

Q: Do I have to take glibc offline to apply the patch?

It depends on the deployment. High-availability or clustered installs can usually patch one node at a time with no full outage. Standalone installs typically need a short restart. Always follow the vendor's documented upgrade steps.

Q: What if my vulnerability scanner still flags CVE-2026-3904 after I patch?

Re-run the scan after a service restart, then confirm the scanner's plugin set is up to date. Some scanners detect by banner version only and lag the official fix metadata by a release.

Other vulnerabilities in the same area that are worth patching alongside this one:

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How to Fix CVE-2026-29050: CWE-22: Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal') — CWE-22: Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')
How to Fix CVE-2026-33216: Path Traversal in nats-server , Path Traversal in nats-server
How to Fix CVE-2026-24951: Critical Vulnerability in myCred , Critical Vulnerability in myCred
How to Fix CVE-2026-41380: CWE-807 Reliance on Untrusted Inputs in a Security Decision in OpenClaw , CWE-807 Reliance on Untrusted Inputs in a Security Decision in OpenClaw

*By Sai Kiran Pandrala*

⚡ At a glance


Severity	CVSS 6.2, Medium
Actively exploited?	No
Affected	the Gnu C Library glibc (2.35 < 2.37)
Fixed in	2.37
Type (CWE)	CWE-366: CWE-366 Race condition within a thread

What is CVE-2026-3904?

Calling NSS-backed functions that support caching via nscd may call the

nscd client side code and in the GNU C Library version 2.36 under high

load on x86_64 systems, the client may call memcmp on inputs that are

concurrently modified by other processes or threads and crash. The nscd client in the GNU C Library uses the memcmp function with

inputs that may be concurrently modified by another thread, potentially

resulting in spurious cache misses, which in itself is not a security

issue. However in the GNU C Library version 2.36 an optimized

implementation of memcmp was introduced for...

In practical terms, a successful attacker gets compromise of the affected component as described in the vendor advisory. There is no confirmed in-the-wild exploitation listed in CISA's KEV catalog at the time of writing, but the CVSS rating still warrants prompt patching.

Am I affected?

You're affected if you run the Gnu C Library glibc at any version in the Affected row above. Use these probes to find your installed build:


# Confirm the installed version via your package manager
dpkg -l | grep -i glibc   # Debian/Ubuntu
rpm -qa | grep -i glibc   # RHEL/CentOS/Rocky

How to fix CVE-2026-3904

The primary fix is to upgrade to the patched build listed in the Fixed in row above (2.37). Pick the platform that matches your install and run the commands below.

Linux (Ubuntu / Debian)


sudo apt-get update
sudo apt-get install --only-upgrade glibc
# Confirm the installed version meets or exceeds 2.37
dpkg -s glibc | grep ^Version

Linux (RHEL / CentOS / Rocky)


sudo dnf upgrade --security glibc -y
rpm -q glibc

Windows (PowerShell, admin)


# Try winget first
winget upgrade --id 'the Gnu C Library.glibc' --silent --accept-source-agreements --accept-package-agreements
# If winget does not know the product, download the patched installer from the vendor and:
Start-Process -FilePath "$env:TEMP\glibc-2.37.msi" -ArgumentList '/qn /norestart' -Wait

PowerShell script (Windows) - detect, back up, upgrade, verify, log


# Run as Administrator
$ErrorActionPreference = 'Stop'
$log = "$env:ProgramData\glibc-Patch-CVE-2026-3904.log"
function Write-Log($msg) { "$(Get-Date -Format s)  $msg" | Tee-Object -FilePath $log -Append }

Write-Log "Starting CVE-2026-3904 remediation for the Gnu C Library glibc"

# 1. Detect: replace the path/version probe with one valid for your install
$installed = (Get-WmiObject -Class Win32_Product |
    Where-Object { $_.Name -like '*glibc*' } |
    Select-Object -First 1 -ExpandProperty Version)
Write-Log "Detected version: $installed"

if (-not $installed) {
    Write-Log "Product not installed on this host; nothing to do."
    return
}
if ([version]$installed -ge [version]'2.37') {
    Write-Log "Already at fixed version $installed; no action needed."
    return
}

# 2. Backup configuration to a timestamped folder
$backup = "$env:ProgramData\glibc-Backup-$(Get-Date -Format yyyyMMdd-HHmm)"
New-Item -ItemType Directory -Path $backup -Force | Out-Null
# Adjust the source path to match your install
$src = "$env:ProgramFiles\the Gnu C Library\glibc"
if (Test-Path $src) { Copy-Item -Path $src -Destination $backup -Recurse -Force }
Write-Log "Backed up config to $backup"

# 3. Apply the patched installer (place the verified file on a share or staging path)
$installer = "$env:TEMP\glibc-2.37.msi"
if (-not (Test-Path $installer)) {
    throw "Patched installer not found at $installer. Stage it from your software repo first."
}
Start-Process msiexec.exe -ArgumentList "/i `"$installer`" /qn /norestart" -Wait
Write-Log "Installer finished"

# 4. Verify
$verify = (Get-WmiObject -Class Win32_Product |
    Where-Object { $_.Name -like '*glibc*' } |
    Select-Object -First 1 -ExpandProperty Version)
if ([version]$verify -ge [version]'2.37') {
    Write-Log "SUCCESS: now at $verify (>= 2.37)"
} else {
    Write-Log "FAILURE: still at $verify after install"
    exit 1
}

Bash script (Linux) - detect, back up, upgrade, verify, log


#!/usr/bin/env bash
set -euo pipefail
LOG=/var/log/glibc-patch-cve-2026-3904.log
log()  { echo "$(date -Iseconds)  $*" | tee -a "$LOG"; }

log "Starting CVE-2026-3904 remediation for the Gnu C Library glibc"

# 1. Detect installed version (works for deb and rpm packages)
if command -v dpkg >/dev/null && dpkg -s glibc >/dev/null 2>&1; then
    CURRENT=$(dpkg-query -W -f='${Version}' glibc)
    PKG_MGR=apt
elif command -v rpm >/dev/null && rpm -q glibc >/dev/null 2>&1; then
    CURRENT=$(rpm -q --queryformat '%{VERSION}' glibc)
    PKG_MGR=dnf
else
    log "glibc not installed via apt or rpm; check your package manager or vendor instructions."
    exit 0
fi
log "Detected: glibc=$CURRENT (manager=$PKG_MGR)"

# 2. Backup config
BACKUP=/var/backups/glibc-$(date +%Y%m%d-%H%M)
mkdir -p "$BACKUP"
for d in /etc/glibc /etc/${pkg%%-*} ; do
    [ -d "$d" ] && cp -a "$d" "$BACKUP/" && log "Backed up $d to $BACKUP"
done

# 3. Upgrade
if [ "$PKG_MGR" = apt ]; then
    sudo apt-get update -y
    sudo apt-get install --only-upgrade -y glibc
else
    sudo dnf upgrade --security -y glibc
fi

# 4. Verify
if [ "$PKG_MGR" = apt ]; then
    NEW=$(dpkg-query -W -f='${Version}' glibc)
else
    NEW=$(rpm -q --queryformat '%{VERSION}' glibc)
fi
log "After upgrade: $NEW"

# Optionally compare against 2.37 with dpkg --compare-versions or sort -V
log "Done. Restart the affected service if the package install did not."

If you can't patch immediately

These are runnable hardening commands. They reduce blast radius but they're not a replacement for the vendor patch.

Rate-limit and watchdog the affected service

Linux:


# Drop traffic above 50 connections/second from a single source
sudo iptables -A INPUT -p tcp --dport 443 -m conntrack --ctstate NEW -m limit --limit 50/s -j ACCEPT
sudo iptables -A INPUT -p tcp --dport 443 -m conntrack --ctstate NEW -j DROP

Set systemd to auto-restart the service on crash:


[Service]
Restart=always
RestartSec=5s

How to verify the fix worked

Run the version probe again and confirm the running build matches the Fixed in row above.


# Confirm the running build matches the patched version listed by the vendor
# Example for Linux package installs:
dpkg -l | grep -i "glibc"   # Debian/Ubuntu
rpm -qa | grep -i "glibc"   # RHEL/CentOS/Rocky

Expected output: the package version should meet or exceed 2.37.

Then re-run any vulnerability scanner you used previously and confirm the finding for CVE-2026-3904 has cleared. Sweep your logs for the indicators of compromise listed in the vendor or CISA advisory, especially if the system was internet-reachable during the disclosure window.

Frequently asked questions

Is CVE-2026-3904 being actively exploited?